Work machines may sometimes include one or more caster wheels which are carried by a machine frame and rotate about a generally vertical axis. The caster wheel assembly typically includes a shaft defining an axis of rotation, a fork rigidly attached to the bottom end of the shaft, and a caster wheel coupled with the distal ends of the fork. A windrower is an example of such a work machine.
Self-propelled windrowers are typically configured with large drive tires in the front and smaller castered tires in the rear. A dual-path hydrostatic system controls both propulsion and direction by controlling the speed and differential speed, respectively, of the front drive wheels. The cutting platform extends crosswise in front of the drive tires.
Ground speeds in the field are increasing. Cutting speeds with rotary cut platforms are often in the 10 to 12 mph range or higher. These ground speeds result in an increasingly rough ride for both the operator and the machine. Larger drive tire sizes offer ride improvement for the front of the machine, but do not address ride issues resulting from the smaller rear tires.
Fatigue of machine components is an additional concern. The rear axle is of particular concern. The rear axle typically consists of an adjustable length beam with a caster-mounted wheel pivoted vertically at each end. The adjustable length of the axle allows the tread width to be adjusted to meet varied needs. The axle is mounted on the machine such that it can pivot horizontally within limits about a point at the midpoint of the beam to allow the rear wheels to conform to uneven ground. The axle itself and the center pivot assembly must withstand horizontal and vertical loads imparted by the caster wheels. These loads increase in magnitude and frequency as ground speeds increase. Loads also increase as tread width is increased to accommodate large windows.
Clearance under the machine must also be adequate to accommodate large windows. The lowest machine clearance is often under the rear axle. The ability to increase this clearance is limited by the pivoting rear axle requirement because the axle must have room to move under the machine frame. One configuration gains clearance by mounting the axle on a cantilevered pivot outside the frame at the rear of the machine. While this configuration increases clearance under the machine, structural and machine length considerations make it undesirable.
It is known to utilize a rear axle suspension on a windrower. In this configuration, the rear axle consists of two, separate beams—one for each caster wheel. The beams are independently pivoted at a point near the centerline of the machine such that each can pivot horizontally within limits. Caster wheels are mounted at the ends of each adjustable length axle, similar to the non-suspended configuration described above. Suspension is provided by placing a pressure-adjustable air bag between a bracket mounted on the frame of the machine and the top of each axle.
While this solution provides a suspension at the rear of the machine, it has the disadvantages of allowing excessive motion at the operator station because there is no inherent damping in the air bag, it changes the castor pivot orientation as it moves through the suspension range, and does not optimize the suspension characteristics by minimizing the unsprung mass. Further, this solution does not increase the clearance under the machine, nor does it diminish the structural concern at the axle pivots.
What is needed in the art is a work machine with castered rear wheels which provide a smooth ride and sufficient ground clearance.